To record or amplify sound events, it is first necessary to convert the sound events into an electrical signal by means of an electro-acoustical converter. The thus gained electrical signal is either amplified further and converted back by another electro-acoustical converter so that the amplified sound signal can be recovered, or it is first stored temporarily in a data carrier before it is converted back into a sound signal.
The electrical channel contained between the two signal converters changes the signal inasmuch as it adds at least some noise to the signal. On the one hand the noise is composed of thermal noise from the structural units, and therefore depends exclusively on temperature when an internal resistance is specified, and on the quantization noise if the electrical signal is converted or reconverted from the analog to the digital form. The signal amplitude of the noise signal cited last is also independent of the amplitude of the useful signal or the audio signal.
The components of the noise signal do not affect the ear as long as they remain significantly below the amplitude of the useful signal. The ear has the ability of fading out signals if their amplitude remains clearly below that of a signal with greater amplitude. Aside from its dependence on amplitude, this fade-out effect is frequency-dependent as well.
Many attempts have been made for that reason in the past to utilize this property of the human hearing, in order to subjectively improve the signal-to-noise ratio.
One of the oldest methods for the subjective improvement of the signal-to-noise ratio consists in dividing the audio-frequency into two channels, where the frequency limit is about 8 kHz, and to reduce the amplification in the channel with the higher frequency jump to zero when the useful signal therein falls below a predetermined threshold.
The unpleasant side effect of this comparatively very simple circuit rests in the significant deterioration of the tone quality, because many overtones occur in the frequency range above 8 kHz, which are uniformly cut off with this method.
Another method which also operates with only two channels, wherein the channel with the higher frequency is affected, is described in the essay "The SSM 2000 HUSH Noise Reduction System" from Analog Devices. This system operates with a variable limiting value which, when exceeded, also controls the high-frequency channel disturbed by the noise.
This system also affects the tone quality audibly in an undesired manner, aside from the fact that pumping effects can occur if the transient period is not fast enough.
With the noise filter according to U.S. Pat. No. 3,803,357 the lower and the upper frequency ranges are transmitted without change, while the middle frequency range is assigned to a multichannel system. This multichannel system contains a number of narrow-band band-pass filters which cover the entire frequency range between the low-pass and the high-pass filter. Each of the narrow-band band-pass filters is followed by a linear amplifier, whose amplification ratio is adjustable. The control of the amplification takes place with the help of a noise tracker which samples the composite signal at the input of all band-pass filters, and equally controls all of the narrow-band channels.
Control of the amplification in the known system includes defining a gap in the vicinity of the zero passage of the audio signal, during which the signal is zero at the output. In this way input signals falling within this gap are not amplified. Sinusoidal and sine-shaped signals whose peak-to-peak amplitude is greater than this dead zone are allowed to pass, but change into a clear step or a bend because of the missing amplification in the vicinity of the zero passage. The nonlinear distortion created in this manner is compensated by a further narrow-band filter at the output of the amplifier.
The cost is comparatively high and the system furthermore produces additional harmonics which become noticeable as an increased disturbance factor, similar to push-pull amplifier stages with transitional distortions.
A further problem with this system are the transient effects, which are known to last longer the narrower the band of the filter is. The results are transitional distortions which are just as undesirable.
U.S. Pat. No. 5,067,157 describes a noise suppression system wherein the audio signal is also divided into several narrow-band channels, and the respective channel is switched on or off as a function of amplitude of the channel signal.
Such a hard switching system tends to have audible pumping effects.